Process for preparing carbon nanotubes

ABSTRACT

Carbon nanotubes are formed on carbon paper by first depositing a metal catalyst on the carbon paper, and passing a feedstock gas containing a source of carbon over the substrate while applying an electrical current thereto to heat the substrate sufficiently to generate a reaction between the catalyst and the feedstock gas. Alternatively, inert gas under pressure is passed through a tubular metal cathode while passing an electric current through the cathode to produce a plasma of fine catalyst particles which are deposited on a porous carbon substrate, and a feedstock gas containing a source of carbon is passed over the substrate to cause a reaction between the catalyst and the carbon source resulting in the formation of carbon nanotubes.

This application claims the benefit of Provisional Application No.60/203,398, filed May 11, 2000.

This invention relates to a process and apparatus for producing carbonnanotubes and nanofibers

During the past eight years carbon nanotubes have generated asubstantial amount of interest in the scientific community. The highchemical stability, mechanical strength and electrical conductivity ofsingle walled (SWCNT) and multiwalled carbon nanotubes (MWCNTs) make thematerial attractive for industrial applications such asmicroelectronics, the strengthening of composite materials and hydrogenstorage.

At present, carbon nanotubes are produced by laser ablation, ablationusing an electric arc and carbon vapor deposition. Examples of thesemethod are described in American Scientist 85 (1997) 324–337; Journal ofMolecular Catalysts A: Chemical 116 (1997) 397–403; Nature 354 (1991)56; Nature 358 (1992) 220; Nature 363 (1993) 603; Chemical Phys. Lett.289 (1998) 602; Appl. Phys. Left 75 (1999) 367; Carbon 35 (1997) 1495;Published International Patent Applications WO 99/06618, Feb. 11, 1999and WO 99/25652, May 27, 1999 and U.S. Pat. No. 5,780,101, issued Jul.14, 1998 and U.S. Pat. No. 5,965,267, issued Oct. 12, 1999.

The first two methods, i.e. laser and electric arc ablation are based onthe same principle, namely the evaporation of a graphite rod enrichedwith a metallic catalyst, and condensation of the vapor thus produced innanotube form. Such methods, which permit the fabrication of bulkmultilayer nanotubes, require large amounts of energy for theevaporation process as opposed to the carbon vapor deposition (CVD)method. In fact, in order to achieve atomic vaporization of one mole ofcarbon, 717 kJ is required, while with the CVD method 227 kJ isgenerated by the decomposition of one mole of acetylene molecules. Withthe CVD method, it is possible to control the area where nanotubes aredeposited on a surface [J. Am. Chem. Soc. 121(1999) 10832 and Science282 (1998) 1105]. The problems associated with the CVD method includelow yield and the generation of large quantities of amorphous carbonduring pyrolysis of the carbon containing gas.

An object of the present invention is to provide a new carbon vapordeposition method which is capable of producing carbon nanotubes in theabsence of amorphous carbon at low heating power.

It will be noted that whereas in the process of the present inventiononly the substrate is heated, in the conventional CVD method the entirereaction vessel is heated.

Another object of the invention is to provide an apparatus for carryingout the above defined process for producing carbon nanotubes.

Accordingly, the present invention relates to a process for producingcarbon nanotubes comprising the steps of:

-   -   (a) depositing a metallic catalyst on a porous carbon substrate;    -   (b) passing a feedstock gas containing a source of carbon        through the substrate; and    -   (c) applying an electrical current to said substrate to heat the        substrate sufficiently to generate a reaction between said        catalyst and said gas, resulting in the formation of carbon        nanotubes.

According to another aspect, the invention relates to an apparatus forproducing carbon nanotubes comprising:

-   -   (a) tube means defining a reaction chamber for receiving a        hydrocarbon gas;    -   (b) spaced apart electrode means in said tube means for        retaining said carbon substrate in said container means; and    -   (c) conductor means for supplying electrical power to said        electrode means for heating said carbon substrate by the Joule        effect, whereby, when the hydrocarbon gas is introduced into        said reaction chamber and the carbon substrate is heated by        introducing electrical power thereto, nanotubes are produced on        said carbon substrate.

The invention is described below in greater detail with reference to theaccompanying drawings, wherein:

FIG. 1 is a schematic block diagram of an apparatus for carrying out theprocess of the invention;

FIG. 2 is an electron micrograph of a typical area of nanotubes producedon a carbon paper substrate using the apparatus of FIG. 1;

FIG. 3 is an electron micrograph of the free ends of nanotubes formed inthe apparatus of FIG. 1;

FIG. 4 is an electron micrograph of the bottom ends of nanotubes formedin the apparatus of FIG. 1;

FIG. 5 is an electron micrograph of a silicate at the base of nanotubesof FIGS. 2 to 4;

FIG. 6 is a schematic block diagram of a second embodiment of theapparatus for carrying out the process of the invention;

FIG. 7 is an electron micrograph of carbon nanotubes formed in theapparatus of FIG. 6;

FIG. 8 is a schematic block diagram of another embodiment of theapparatus for carrying out the process of the invention;

FIG. 9 is an electron micrograph of carbon nanotubes formed in theapparatus of FIG. 8; and

FIG. 10 is a schematic block diagram of another apparatus for carryingout the process of the invention.

EXPERIMENTAL

Method 1

3 ml of a 1.5M aqueous ferrous nitrate solution is added to a solutioncontaining 2 mL of tetraethyl orthosilicate and 2 mL of ethanol. Then0.5 mL of 10% hydrogen fluoride acid is added to the ferrousnitrate/orthosilicate solution to give a gel containing the ferrouscatalyst. A strip of carbon paper is soaked for 2 minutes in the gel.The carbon paper is then blotted and dried for 5 minutes at roomtemperature. A thin film of silicate containing homogeneously dispersedferrous nitrate is thus deposited on the carbon paper. Nanoscopic ironcatalyst particles produced by this process are now available for thesynthesis of nanotubes.

The carbon paper 1 (FIG. 1) carrying the catalyst is placed between twographite electrodes 2, mounted in a quartz tube 3. Stainless steel rods4 connected to the electrodes 2 supply electrical power to theelectrodes for heating the carbon paper 1 by the Joule effect. In thiscase, the power is 40 to 100 watts (5 to 8 amperes). A current of a fewamperes is sufficient to raise the temperature of the carbon paper up to1000° C. in less than 1 second. The carbon paper is thus heated in afirst step to 200–600° C., preferably 400° C. in an argon atmosphere todecompose nitrates into oxide, which is then reduced to the metal formin an argon/hydrogen atmosphere. The iron catalyst is dispersed in thesilicate. In order to obtain the carbon nanotubes, a mixture of 90%argon, 5% hydrogen and 5% of a hydrocarbon gas (acetylene or ethylene)is caused to flow over the paper for 15 minutes.

Carbon nanostructures were produced on the following metals: Fe Co, Ni,their binary alloys Fe_(0.5)Co_(0.5), Fe_(0.5)Ni_(0.5) andNi_(0.5)Co_(0.5), and their ternary alloy Fe_(0.33)CO_(0.33)Ni_(0.33)The gases used include acetylene, ethylene, methane and CO at theiroptimum pyrolysis temperatures of 600 for acetylene and 700 for theother gases.

The samples were analyzed using a transmission electron microscope (TEM)and a high resolution transmission electron microscope (HRTEM). FIG. 2illustrates a typical area of nanotubes produced on the carbon paper 1.The carbon paper included carbon fibers with a diameter of 5 μm coveredwith the silicate. An excess of the silicate in the form of flakes isalso present on the substrate. As shown in FIGS. 3 and 4, the nanotubesgrow from a particle of metallic catalyst which is on the silicate. Withthe carbon paper substrate used, the contrast between the nanotubes(mainly transparent) and the metallic catalyst particles is easily seenby TEM. Thus, it has been found that the catalyst particles are foundboth at the base and at the tips of the nanotubes.

It is readily apparent from FIG. 5 that there is no amorphous carbon onthe substrate, in the silicate or on the nanotubes. The types ofnanotubes (mostly multi-wall) produced are similar to those obtained bythe conventional CVD method. The tubes have an external diameter of 5–25nm, with 10–20 distinct layers when ethylene or acetylene is used as thecarbon source. It has been observed that 15 minutes after the beginningof the synthesis process the nanotubes obtained had a length of 1–10 μmwith ethylene and below 1 μm with acetylene. Iron at 700° C. is morereactive than at 600° C., and thus more readily decomposes thehydrocarbon to form longer tubes, i.e. the kinetics of nanotubeformation is influenced more by temperature than by the type of gas usedas the carbon source.

It is observed from FIG. 5 that iron particles with a diameter greaterthan 30 nm are entirely surrounded by graphite carbon, while catalystparticles having a diameter less than 25 nm lead to nanotube formation.Thus, it is concluded that only catalyst particles having a diameterless than 25 nm lead to the growth of nanotubes. Larger catalystparticles actively become surrounded by carbon. This observation leadsto the conclusion that the main catalytic phenomena is a surface processand not a process occurring inside of the catalyst. The catalyticdecomposition of the carbon source gas at the surface of the catalystparticles creates a reactive layer. The thickness of the gas layerdepends on the quantity of gas and the reactivity of the catalystparticles. The reactivity is dependent upon the type of metal and thereaction temperature. The gas layer leads to the growth of nanotubesonly if the force leading to growth are strong than the force retainingthe gas at the surface of the catalyst particles.

Method 2

In a second process in accordance with the present invention, thecatalyst, in the form of a suspension of fine metallic particles, issprayed onto carbon paper 1 (FIG. 6).

The catalyst is produced by suspending ultrafine metal particles in asuitable solvent such as toluene by adding a surfactant. The metalcatalyst particles are selected from the group consisting of Fe, Fe₂O₃,Ni, Co and alloys of Fe, Ni and Co having a particle size of 2 to 100nm, ideally 2 to 5 nm. Suitable surfactants include (octylphenoxy)polyethoxy ethanol, N′-cyclohexyl-p-phenylenediamine and preferablydioctyl sulfosuccinate sodium salt. By using an ultrasonic bath, thesuspension is made homogeneous. The resulting suspension is sprayeddirectly onto the carbon paper 1 using a conventional chromatographysprayer. For example, 1–20 mg of fine catalyst particles are added to 5ml of toluene containing 1–20 mg of the surfactant, and the suspensionis sprayed onto the carbon paper 1.

The carbon paper 1 (FIG. 6) carrying the catalyst 1 is placed betweentwo graphite electrodes 2 mounted in a quartz tube 3, and electricalpower is supplied to the electrodes via stainless steel rods 4, asdescribed in Method 1. The mixture of argon, hydrogen and hydrocarbongas is fed into the tube 3 and through the carbon paper to producenanotubes

The samples were analyzed using a scanning electron microscope (SEM).FIG. 7 illustrates a typical area of nanotubes produced on the carbonpaper 1. The nanotubes are seen mainly in the center of FIG. 7, theremainder of the figure showing larger carbon fibers from the carbonpaper.

Method 3

In a third method fine catalyst particles are formed using a hollowcathode discharge apparatus, the principle of which is described inarticles by K. Ishii in J. Vac. Sci Technol. A7(2), p 256–258, 1989 andby K. Ishii et al in J. Vac. Sci. Technol. A17(1), P 310–313, 1999.

Referring to FIG. 8, the hollow cathode apparatus includes a housing 6containing a tubular metal cathode 7 in the bottom end thereof, whichalso serves as a gas inlet. The metal used in the cathode 7 is iron forproducing a catalyst of fine iron particles. The cathode 7 can also be amolybdenum tube for producing a molybdenum catalyst, an iron tube with ashorter cobalt tube welded to the interior thereof for producing aniron/cobalt alloy, or an iron tube with shorter nickel and cobalt tubeswelded therein, i.e. slid into the iron tube and welded thereto forproducing an Fe/Ni/Co catalyst. Gas is discharged from the housing 6 viaan outlet 12 and a pump (not shown).

In a first step, argon gas flows into the cathode 7 at a pressure of0.2–1 torr and a flow rate of 100–500 cm³/min. Electrical power (DC at100 MA to 1 A and a voltage of approximately 400 volts) from a source 8is supplied to the cathode 7 and an annular anode 9 via lines 10 and 11.A sputtering of small particles of metal catalyst (plasma 13) occurs inthe discharge region of the cathode 7 within a few seconds of turning onthe power. When the iron cathode contains Co or Co and Ni, metal ions inthe plasma 13 combine to form alloys. The proportions of the metals inthe alloy are dictated by the exposed area of the Co or Co and Ni tubesin the cathode 7. The metal ions flow through the anode 9 in thedirection of arrows A and are deposited on carbon paper 1 mounted in thehousing 6. The size of the catalyst particles is controlled by thecurrent and voltage applied to the cathode 7, and by the pressure andrate of gas flow through the cathode 7. In a second step, power flow tothe cathode 7 is discontinued, and a mixture of argon and a source ofcarbon is fed through the cathode 7 to the heated carbon paper withcatalyst thereon.

Suitable carbon sources include carbon monoxide, methane, ethane andethylene.

It is possible to produce the catalyst and fabricate nanotubes in asingle operation by introducing a mixture of argon and a carbon sourcethrough the cathode simultaneously. However, a one-step method couldresult in the formation of amorphous carbon which is undesirable in theproduction of nanotubes.

When nickel was used as the hollow cathode, and CO as the carbon sourcein a two-stage process, the result was the nanotubes illustrated in FIG.9.

Method 4

With reference to FIG. 10, in a fourth method in accordance with theinvention an auxiliary plasma is produced to prevent coalescing ofcatalyst particles as they are discharged from the cathode.

In the apparatus of FIG. 10, argon is introduced into the bottom end ofa tube 15 via an inlet 16. The argon gas is discharged through a tubularcathode 18 in the open top end of the tube 15 producing plasma in anarea beneath carbon paper 1. The tube 15 passes through the otherwiseclosed bottom end 23 of a second larger tube or housing 24. The tube 15is connected to a microwave source (not shown) by a co-axial connector25. Power is supplied to the cathode 18 and an anode 28 from a DC source29 via leads 30 and 31. Lead 30 is grounded at 33, and is connected tothe cathode 18 via the tubes 24 and 15.

In operation, argon is introduced into the tube 15 via the inlet 16 asdescribed in Method 3. When power is supplied to the cathode 18,sputtering occurs to produce ultrafine catalyst particles, which aredeposited on the carbon paper 1. The microwaves produced in the tube 15create an auxiliary plasma 34 between the top end of the tube 15 and thecarbon paper 1 which ensures the production of ultrafine particles ofcatalyst. The argon is introduced into the tube 15 at a flow rate of100–500 cm³/min and a pressure of 0.2–1 torr. The DC current to thecathode 18 is 100 mA to 1 A and the voltage is 400 volts.

As soon as the catalyst has been deposited on the carbon paper 1,carbon-containing gas is introduced into the tube 15 via the inlet 16which results in the formation of nanotubes on the hot carbon paper. Asin the case of Method 3, the nanotubes can be formed in a single step.In the one step method, argon under pressure is introduced into the tube15 to produce catalyst particles which are deposited on the carbon paper1. The carbon-containing gas is simultaneously introduced into the tube15.

1. A process for producing carbon nanotubes comprising the steps of: (a)depositing a metallic catalyst on a porous carbon substrate; (b) passinga feedstock gas containing a source of carbon through the substrate; and(c) applying an electrical current to said substrate to heat thesubstrate sufficiently to generate a reaction between said catalyst andsaid gas, resulting in the formation of carbon nanotubes.
 2. The processof claim 1, wherein said substrate is carbon paper.
 3. The process ofclaim 2, wherein said catalyst is selected from the group consisting ofiron, nickel and cobalt.
 4. The process of claim 3, wherein the sourceof carbon in said feedstock gas is selected from the group consisting ofcarbon monoxide, acetylene, ethylene and methane.
 5. The process ofclaim 4, wherein feedstock gas includes hydrogen and an inert gas. 6.The process of claim 5, wherein said inert gas is argon.
 7. A processfor producing carbon nanotubes comprising the steps of: (a) passing aninert gas under pressure through a tubular metal cathode while passingan electric current through the cathode to produce a plasma of finecatalyst particles; (b) depositing the catalyst particles thus producedonto a porous carbon substrate proximate the cathode whereby thesubstrate is heated; and (c) passing a feedstock gas containing a sourceof carbon through the heated substrate to cause a reaction between thecatalyst and the carbon source resulting in the formation of carbonnanotubes.
 8. The process of claim 7, wherein the inert gas containingthe catalyst and the feedstock gas are simultaneously passed through thecathode to form the nanotubes in one step.
 9. The process of claim 7,wherein the inert gas containing the catalyst and the feedstock gassimultaneously impinge upon the carbon substrate to form the nanotubesin one step.
 10. The process of claim 9, wherein the inert gas is argonand the feedstock gas is selected from the group consisting of methane,ethane, acetylene and carbon monoxide.
 11. The process of claim 7,wherein the tubular cathode is formed of an element selected from thegroup consisting of iron, nickel and cobalt.
 12. The process of claim 7,wherein the tubular cathode is iron and at least one of cobalt andnickel, whereby the catalyst produced by passing the inert gas throughthe cathode is an alloy.
 13. The process of claim 7, including the stepof producing an auxiliary plasma in said cathode to prevent coalescingof catalyst particles produced by passing said feedstock gas throughsaid cathode.
 14. An apparatus for producing carbon nanotubescomprising: (a) tube means defining a reaction chamber for receiving ahydrocarbon gas; (b) spaced apart electrode means in said tube means fortogether retaining a carbon substrate in said tube means; and (c)conductor means for supplying electrical power to said electrode meansfor heating said carbon substrate by the Joule effect, whereby, when thehydrocarbon gas is introduced into said reaction chamber and the carbonsubstrate is heated by introducing electrical power thereto, nanotubesare produced on said carbon substrate.
 15. An apparatus for producingcarbon nanotubes comprising: (a) a housing defining a reaction chamberfor receiving a carbon substrate; (b) an inlet tube in said reactionchamber for introducing gas into said housing; (c) a catalytic metalcathode in said inlet tube; (d) an anode in said reaction chamberbetween and spaced apart from said cathode and a carbon substrate insaid housing, whereby, when electric current is supplied to the anodeand an inert gas is passed through said cathode under pressure, aprimary plasma of fine metallic catalyst particles are produced anddeposited on said carbon substrate, and, when a gas containing a sourceof carbon is passed through said inlet tube and impinges on saidsubstrate, carbon nanotubes are formed.
 16. The apparatus of claim 15,wherein said cathode defines the entire inlet tube.
 17. The apparatus ofclaim 15, wherein said anode is coaxial with said cathode in said inlettube.
 18. The apparatus of claim 17, including a cable in said tube forconnecting the tube to a source of microwave energy, whereby, whenmicrowave energy is supplied to the cable and the inert gas is passedthrough the inlet tube, an auxiliary plasma is produced between saidtube and the carbon substrate to prevent coalescing of catalystparticles.